Ionic liquid for pickling stainless steel and method for pickling stainless steel by using same

ABSTRACT

Disclosed are an ionic liquid for pickling a stainless steel capable of rapidly removing oxide scale from the stainless steel at room temperature without using nitric acid or hydrofluoric acid and a method for pickling a stainless steel by using the same. The method for pickling a stainless steel according to an embodiment includes performing electrolytic pickling treatment by immersing a stainless steel in a pickling solution including an ionic liquid, wherein the ionic liquid comprises at least one of an imidazolium cation, a betainium cation, a sulfonium cation, a piperidinium cation, a phosphonium cation, an ammonium cation, a pyridium cation, a pyrrolidinium cation, and a morpholinium cation, as a cationic functional group, and at least one of a halide anion, a sulfonate anion, an alkylsulfate anion, a phosphinate anion, a salicylate anion, a nitrate anion, a tetrafluoroborate anion, a hexafluorophosphate anion, and a bistriflimide anion, as an anionic functional group.

TECHNICAL FIELD

The present disclosure relates to an ionic liquid for pickling astainless steel and a method for pickling a stainless steel by using thesame, and more particularly, to an ionic liquid for pickling a stainlesssteel capable of rapidly removing oxide scale from the stainless steelat room temperature without using nitric acid or hydrofluoric acid and amethod for pickling a stainless steel by using the same.

BACKGROUND ART

Cold-rolled stainless steel sheets undergo a heat treatment process at atemperature of 800 to 1150° C. to obtain mechanical properties aftercold rolling. In this case, oxide scale (SiO₂, (Cr,Fe)₃O₄) having athickness of 100 to 300 nm, or the like is generally formed by reactionbetween high-temperature oxygen in a furnace and the surface of thecold-rolled steel sheet, and thus surface quality deteriorates andcorrosion resistance decreases.

Therefore, a pickling process to remove such oxide scale is conducted inorder to obtain a cold-rolled stainless steel sheet having high surfacequality and excellent corrosion resistance. In general, the picklingprocess is performed by using various methods, alone or in combination,such as physical descaling (brushing and shot ball blasting),electrolytic descaling (sodium sulfate, sulfuric acid, or nitric acidelectrolyte), and chemical descaling (salt bath and mixed acid bath).

A conventional pickling process is generally performed in two steps. Aprimary pickling step to remove oxide scale is conducted by immersing astainless steel in a solution containing sodium sulfate electrolyte andapplying a current thereto or by immersing the stainless steel in a saltbath containing sodium hydroxide and sodium nitrate at a hightemperature of 400° C. or higher, and then a secondary pickling step isperformed by using nitric acid or a mixed acid containing nitric acidand hydrofluoric acid at a high temperature of 80° C. or higher toobtain high surface quality and corrosion resistance by uniformlyforming a passivated layer. In this regard, nitric acid has effects onincreasing the activity of hydrofluoric acid by lowering a pH in apickling bath and maintaining an oxidation-reduction potential suitablefor pickling by oxidizing ferrous cation (Fe²⁺) dissolved on the surfaceof the steel sheet to ferric cation (Fe³⁺).

However, nitric acid causes various environmental problems. The use of asalt containing nitric acid and nitric acid generates NO_(x) that is asubstance whose emission into the atmosphere is regulated and nitratenitrogen (NO₃—N) is contained in waste acid and wash water. In addition,there may be a problem that production costs are considerably increaseddue to additional installation of facilities to prevent environmentalpollution in a pickling process and operation costs thereof because atotal nitrogen contained in discharged effluent water is limited and aNO_(x) concentration of a facility discharging to atmosphere is limitedin accordance with domestic and foreign strengthened environmentalregulations.

As a prior art to solve these problems, developed is a nitric acid-freepickling method in which nitric acid is replaced with hydrochloric acid,sulfuric acid, or the like and insufficient oxidizing power is replacedby hydrogen peroxide, potassium permanganate, ferric ion (Fe³⁺), and airinjection during a pickling process.

Starting with Patent Document 1 in which an oxidation-reductionpotential of a pickling solution is maintained at 300 mV or more byusing sulfuric acid, hydrofluoric acid, or iron sulfate as a picklingsolution and adding hydrogen peroxide, Patent Documents 2 and 3 in whichranges of hydrofluoric acid, iron ions, air, hydrogen peroxide, andoxidation-reduction potential (ORP) of a solution are appropriatelymodified, and other patent applications have been continuously filedsince the 1990s. However, most of those methods are limitedly applicableto products that do not strictly require high quality such as wire rods,bar steels, thick plates.

Patent Document 4 discloses a pickling method by using a picklingsolution containing sulfuric acid, hydrofluoric acid, and ferric salt,periodically adding hydrogen peroxide, and adjusting a composition of awetting agent, a polishing agent, and a corrosion inhibitor, wherein thepickling solution is automatically controlled based on Fe(III) and ORPthereby. Accordingly, CLEANOX352, as a pickling solution, has beencommercialized and the most widely used worldwide. Although this methodhas been practically used for wire rods and hot-rolled products,manufacturing costs for products are higher than those of conventionalproducts by 20% or more and the composition of the pickling solution andthe control method are complex. Particularly, due to a relatively lowpickling rate of about 1.5 to 3 g/m²·min, this method is not suitablefor a high-speed pickling (TV=130 or more). In addition, Patent Document5, as a patent disclosing improved properties, discloses a method ofincreasing a pickling rate by adding copper and chlorine ions to apickling composition. However, when an open circuit potential (OCP) ofthe surface of a ferritic stainless steel sheet is less than anoxidation-reduction potential (0.1 V) of copper, copper particles may beprecipitated on the surface of the steel sheet during a picklingprocess, thereby causing discoloration of the steel sheet. In addition,when a pickling solution includes chlorine ions at a certain level ormore, there is a risk of occurrence of pitting corrosion.

As described above, extensive research on the composition of a nitricacid-free pickling solution has been conducted so far. However,according to the conventional pickling processes, a high concentrationpickling solution should be used, pickling should be performed byimmersing a stainless steel in a high-temperature pickling solution, andpickling takes a long time.

(Patent Document 0001) DE Publication No. DE 3937438

(Patent Document 0002) U.S. Pat. No. 5,154,774

(Patent Document 0003) EP Publication No. EP 0236354

(Patent Document 0004) U.S. Pat. No. 5,908,511

(Patent Document 0005) U.S. Pat. No. 6,554,908

DISCLOSURE Technical Problem

The present disclosure has been proposed to solve the above-describedproblems, and provided is an ionic liquid for pickling stainless steelcapable of rapidly removing oxide scale of stainless steel at roomtemperature without using nitric acid or hydrofluoric acid and methodfor pickling stainless steel by using the same.

Technical Solution

In accordance with an aspect of the present disclosure to achieve theabove-described objects, an ionic liquid for pickling a stainless steelmay include at least one of an imidazolium cation, a betainium cation, asulfonium cation, a piperidinium cation, a phosphonium cation, anammonium cation, a pyridium cation, a pyrrolidinium cation, and amorpholinium cation, as a cationic functional group, and at least one ofa halide anion, a sulfonate anion, an alkylsulfate anion, a phosphinateanion, a salicylate anion, a nitrate anion, a tetrafluoroborate anion, ahexafluorophosphate anion, and a bistriflimide anion, as an anionicfunctional group.

The ionic liquid for pickling a stainless steel according to anembodiment of the present disclosure may be represented by one of thecompounds (a) to (c) below.

Also, in accordance with another aspect of the present disclosure toachieve the above-described objects, a method for pickling a stainlesssteel includes performing electrolytic pickling treatment by immersing astainless steel in a pickling solution including an ionic liquid,wherein the ionic liquid includes at least one of an imidazolium cation,a betainium cation, a sulfonium cation, a piperidinium cation, aphosphonium cation, an ammonium cation, a pyridium cation, apyrrolidinium cation, and a morpholinium cation, as a cationicfunctional group, and at least one of a halide anion, a sulfonate anion,an alkylsulfate anion, a phosphinate anion, a salicylate anion, anitrate anion, a tetrafluoroborate anion, a hexafluorophosphate anion,and a bistriflimide anion, as an anionic functional group.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, the ionic liquid may berepresented by one of the compounds (a) to (c) below.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, the ionic liquid may be containedin the pickling solution in a concentration of 2 M or less.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, the pickling solution may be asolution prepared by adding an ionic liquid to a hydrochloric acidsolution having a concentration of 1 M or less or a neutral solutionincluding at least one of sodium chloride, potassium chloride, magnesiumchloride, sodium sulfate, and potassium sulfate.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, a temperature of the picklingsolution may be from 15 to 25° C.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, the electrolytic picklingtreatment may be performed by applying a current density of 0.5 to 1.3A/cm² while maintaining a surface potential of the stainless steel inthe range of −1.5 to 1.5 V using an Ag/AgCl electrode as a referenceelectrode.

In the method for pickling the stainless steel according to anembodiment of the present disclosure, a time taken to completely removethe oxide scale may be 1 minute or less.

Advantageous Effects

According to embodiments of the present disclosure, oxide scale of astainless steel may be removed in an eco-friendly manner by adding anionic liquid to a pickling solution without using nitric acid orhydrofluoric acid.

In addition, according to an embodiment of the present disclosure, thenumber of times of repeatedly applying a potential until the oxide scaleof the stainless steel is completely removed in a state where thestainless steel is immersed in the pickling solution at a temperature of15 to 25° C., rather than a high temperature of 80° C. or higher, may be5 times or less and a time taken therefor may be 1 minute or less.

In addition, according to the present disclosure, oxide scale may beremoved using a low concentration of an acidic solution or neutralsolution, rather than a high concentration, in a pickling solution.

In addition, according to the present disclosure, economic feasibilitymay be improved by simplifying a pickling process conventionallyperformed in two steps into an electrolytic pickling process performedin a single step.

BEST MODE

An ionic liquid for pickling a stainless steel according to anembodiment of the present disclosure may include at least one of animidazolium cation, a betainium cation, a sulfonium cation, apiperidinium cation, a phosphonium cation, an ammonium cation, apyridium cation, a pyrrolidinium cation, and a morpholinium cation, as acationic functional group, and at least one of a halide anion, asulfonate anion, an alkylsulfate anion, a phosphinate anion, asalicylate anion, a nitrate anion, a tetrafluoroborate anion, ahexafluorophosphate anion, and a bistriflimide anion, as an anionicfunctional group.

Modes of the Invention

Hereinafter, preferred embodiments of the present disclosure will now bedescribed. However, the present disclosure may be embodied in manydifferent forms and should not be construed as being limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the disclosure to those skilled in the art.

The terms used herein are merely used to describe particularembodiments. Thus, an expression used in the singular encompasses theexpression of the plural, unless it has a clearly different meaning inthe context. In addition, it is to be understood that the terms such as“including” or “having” are intended to indicate the existence offeatures, steps, functions, components, or combinations thereofdisclosed in the specification, and are not intended to preclude thepossibility that one or more other features, steps, functions,components, or combinations thereof may exist or may be added.

Meanwhile, unless otherwise defined, all terms used herein have the samemeaning as commonly understood by one of ordinary skill in the art towhich this disclosure belongs. Thus, these terms should not beinterpreted in an idealized or overly formal sense unless expressly sodefined herein. As used herein, the singular forms are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

In addition, the terms “about”, “substantially”, etc. used throughoutthe specification mean that when a natural manufacturing and substanceallowable error are suggested, such an allowable error corresponds avalue or is similar to the value, and such values are intended for thesake of clear understanding of the present invention or to prevent anunconscious infringer from illegally using the disclosure of the presentinvention.

Also, as used herein, the term “stainless steel” refers to a cold-rolledstainless steel sheet manufactured by cold rolling and annealingaccording to a process commonly used to manufacture cold-rolledstainless steel sheets. However, the embodiment is not interpreted asbeing limited thereto, and the stainless steel means any stainless steelhaving oxide scale formed on the surface thereof within a range that maybe clearly recognized by those of ordinary skill in the art.

In addition, as used herein, the term “ionic liquid” refers to a saltcompound including a metal cation and a non-metal anion and present as aliquid at a temperature of 100° C. or below.

The present disclosure provides a method for removing oxide scale formedon the surface of the stainless steel by immersing the stainless steelin a pickling solution and performing an electrolytic picklingtreatment. The pickling solution of the present disclosure refers to anacidic solution or neutral solution containing an ionic liquid.

The present inventors have found that efficiency of pickling a stainlesssteel may be increased by adding an ionic liquid to a pickling solution.The ionic liquid according to the present disclosure may include atleast one of an imidazolium cation, a betainium cation, a sulfoniumcation, a piperidinium cation, a phosphonium cation, an ammonium cation,a pyridium cation, a pyrrolidinium cation, and a morpholinium cation, asa cationic functional group, and at least one of a halide anion, asulfonate anion, an alkylsulfate anion, a phosphinate anion, asalicylate anion, a nitrate anion, a tetrafluoroborate anion, ahexafluorophosphate anion, and a bistriflimide anion, as an anionicfunctional group.

In addition, according to an embodiment, the ionic liquid of the presentdisclosure may be represented by one of the compounds (a) to (c) below.

The compounds (a) to (c) are sequentiallytrihexyl(tetradecyl)phosphonium chloride, methyltrioctylammoniumchloride, and 1-butyl-3-methylimidazolium chloride.

When the ionic liquid of the present disclosure is added to a picklingsolution, the ionic liquid performs a function to dissolve a metal of ametal oxide. Therefore, according to the present disclosure, oxide scaleof a stainless steel may be removed in an eco-friendly manner by addingan ionic liquid to a pickling solution without using nitric acid orhydrofluoric acid. Also, the oxide scale of the stainless steel may berapidly removed by immersing the pickling solution at room temperaturerather than a high temperature of 80° C. or higher, and the oxide scalemay be sufficiently removed by using a low concentration, rather than ahigh concentration, of the acidic solution or neutral solution in thepickling solution. In addition, economic feasibility may be improved bysimplifying a pickling process conventionally performed in two stepsinto an electrolytic pickling process performed in a single step.

According to the present disclosure, the ionic liquid is added to thepickling solution. According to an embodiment, the ionic liquid may becontained in the pickling solution in a concentration of 2 M or less.However, in the case where the ionic liquid is added to an acidicsolution, the pickling efficiency may be sufficiently increased in a lowconcentration of 0.2 M or less. In this regard, molarity of the ionicliquid corresponds to a value calculated relative to a total volume ofthe pickling solution.

According to the present disclosure, oxide scale may be sufficientlyremoved in the case of using a low concentration of the acidic solution,rather than a high concentration, by adding the ionic liquid to thepickling solution. According to an embodiment, the acidic solution maybe a hydrochloric acid solution having a concentration of 1 M or less.In this regard, the molarity of the hydrochloric acid solutioncorresponds to a value calculated relative to a total volume of thepickling solution.

The neutral solution according to the present disclosure is notparticularly limited, as long as the neutral solution is suitable forimmersing the stainless steel and the electrolytic pickling treatmentand contains a water-soluble electrolyte commonly used in theelectrochemical field. Examples of the electrolyte may include sodiumchloride (NaCl), potassium chloride (KCl), magnesium chloride (MgCl₂),sodium sulfate (Na₂SO₄), and potassium sulfate (K₂SO₄). In addition, theconcentration of the neutral solution is not particularly limited, aslong as the electrolytic pickling treatment is performed using theneutral solution. The neutral solution according to an embodiment may bea 1 M to 2 M sodium chloride solution.

According to the present disclosure, oxide scale may be rapidly removedby immersing the stainless steel in the pickling solution at atemperature of 15 to 25° C., rather than a high temperature of 80° C. orhigher. According to an embodiment of the present disclosure, the numberof times of repeatedly applying a potential until the oxide scale of thestainless steel is completely removed may be 5 times or less and a timetaken therefor may be 1 minute or less.

The electrolytic pickling treatment according to the present disclosureis a process of removing oxide scale formed on the surface of thestainless steel by performing electrolytic pickling treatment afterimmersing the stainless steel in a pickling solution prepared by addingan ionic liquid to an acidic solution or neutral solution. According toan embodiment, the electrolytic pickling treatment may be performed byapplying a current density of 0.5 to 1.3 A/cm² while maintaining asurface potential of the stainless steel at −1.5 to 1.5 V using anAg/AgCl electrode as a reference electrode. The electrolytic picklingtreatment may be performed by AC electrolysis or DC electrolysis inwhich a currents is applied, or by a constant voltage method in which avoltage is applied.

Hereinafter, the present disclosure will be described in more detailthrough examples. However, it is necessary to note that the followingexamples are only intended to illustrate the present disclosure in moredetail and are not intended to limit the scope of the presentdisclosure. This is because the scope of the present disclosure isdetermined by matters described in the claims and able to be reasonablyinferred therefrom.

Examples

STS 304 type steel containing 18 wt % of Cr and 8 wt % of Ni wascold-rolled and annealed using a process commonly used to manufacturecold-rolled stainless steel sheets. Although the STS 304 type steel wasused in the examples of the present disclosure, any other stainlesssteel having a different compositions and including oxide scale formedon the surface thereof may also be pickled according to the picklingmethod of the present disclosure.

To remove oxide scale formed by annealing heat treatment, the stainlesssteel was immersed in the pickling solution shown in Table 1 below at atemperature of 15 to 25° C., and then electrolytic pickling treatmentwas performed. The electrolytic pickling treatment was performed byrepeatedly applying a potential several times at a scan speed of 0.5 V/suntil the oxide scale is completely removed in a state where the surfacepotential of the stainless steel was maintained in the range of −1.5 to1.5 V using an Ag/AgCl electrode as a reference electrode. The removalof the oxide scale was observed by using an optical microscope and anelectron microscope.

In Table 1 below, the compositions (acidic solution or neutral solution,ionic liquid) of the pickling solution are listed in the order ofmolarity (M) and compounds of compositions. In this regard, the molarityis a value calculated relative to a total volume of the picklingsolution. The current density (A/cm²) of Table 1 means a current densityapplied to the stainless steel, and the number of repetition means thenumber of repeatedly applying a potential until the oxide scale iscompletely removed. The time (sec) of Table 1 means a time taken untilthe oxide scale is completely removed.

TABLE 1 Pickling solution Acidic solution Current No. of or neutralsolution Ionic liquid density repetition Time Molarity Compound MolarityCompound (A/cm²) (times) (sec) Inventive Example 1 1M HCl 0.17M

1.01 4 24 Inventive Example 2 1M NaCl 0.17M

0.492 5 30 Inventive Example 3 1M NaCl 0.17M

0.643 5 30 Inventive Example 4 1M NaCl 0.17M

0.669 5 30

Referring to the results of Table 1, it was confirmed that oxide scaleof the stainless steel may be removed in an eco-friendly manner byadding the ionic liquid to the pickling solution without using nitricacid or hydrofluoric acid according to the present disclosure. Also, itwas confirmed that the oxide scale of the stainless steel may becompletely removed at a high speed within 1 minute by repeatedlyapplying a potential 5 times or less by immersing the stainless steel inthe pickling solution at room temperature, rather than at a hightemperature of 80° C. or higher according to the present disclosure.Also, it was confirmed that oxide scale may be removed by using a lowconcentration of the acidic solution or neutral solution, rather than ahigh concentration, in the pickling solution according to the presentdisclosure. Also, it was confirmed that economic feasibility may beimproved by simplifying a pickling process conventionally performed intwo steps into the electrolytic pickling process performed in a singlestep according to the present disclosure. While the present disclosurehas been particularly described with reference to exemplary embodiments,it should be understood by those of skilled in the art that variouschanges in form and details may be made without departing from thespirit and scope of the present disclosure.

INDUSTRIAL APPLICABILITY

Because the oxide scale of the stainless steel may be rapidly removed atroom temperature in an eco-friendly manner without using nitric acid orhydrofluoric acid according to the present disclosure, an ionic liquidfor pickling a stainless steel and a method for pickling the stainlesssteel using the same may be provided with improved economic feasibility.

1. An ionic liquid for pickling a stainless steel comprising: at leastone of an imidazolium cation, a betainium cation, a sulfonium cation, apiperidinium cation, a phosphonium cation, an ammonium cation, apyridium cation, a pyrrolidinium cation, and a morpholinium cation, as acationic functional group, and at least one of a halide anion, asulfonate anion, an alkylsulfate anion, a phosphinate anion, asalicylate anion, a nitrate anion, a tetrafluoroborate anion, ahexafluorophosphate anion, and a bistriflimide anion, as an anionicfunctional group.
 2. The ionic liquid according to claim 1, wherein theionic liquid is represented by one of the compounds (a) to (c) below:


3. A method for pickling a stainless steel, the method comprising:performing electrolytic pickling treatment by immersing a stainlesssteel in a pickling solution including an ionic liquid, wherein theionic liquid comprises at least one of an imidazolium cation, abetainium cation, a sulfonium cation, a piperidinium cation, aphosphonium cation, an ammonium cation, a pyridium cation, apyrrolidinium cation, and a morpholinium cation, as a cationicfunctional group, and at least one of a halide anion, a sulfonate anion,an alkylsulfate anion, a phosphinate anion, a salicylate anion, anitrate anion, a tetrafluoroborate anion, a hexafluorophosphate anion,and a bistriflimide anion, as an anionic functional group.
 4. The methodaccording to claim 3, wherein the ionic liquid is represented by one ofthe compounds (a) to (c) below:


5. The method according to claim 3, wherein the ionic liquid iscontained in the pickling solution in a concentration of 2 M or less. 6.The method according to claim 3, wherein the pickling solution is asolution prepared by adding an ionic liquid to a hydrochloric acidsolution having a concentration of 1 M or less or a neutral solutionincluding at least one of sodium chloride, potassium chloride, magnesiumchloride, sodium sulfate, and potassium sulfate.
 7. The method accordingto claim 3, wherein a temperature of the pickling solution is from 15 to25° C.
 8. The method according to claim 3, wherein the electrolyticpickling treatment is performed by applying a current density of 0.5 to1.3 A/cm² while maintaining a surface potential of the stainless steelin the range of −1.5 to 1.5 V using an Ag/AgCl electrode as a referenceelectrode.
 9. The method according to claim 3, wherein a time taken tocompletely remove the oxide scale is 1 minute or less.